This invention relates to motor speed control circuits and more particularly to a motor speed control circuit for use in battery-powered applications in which the motor speed control circuit works independently of the voltage delivered by the battery.
One of the most persistent problems in portable tape recorders and record players is the control of the speed of the tape drive or record drive motor. The problem arises essentially because these tape recorders or record players are powered by portable sources of power in which the voltage delivered by these sources varies considerably either due to the loading conditions or due to the running down of the battery.
While in the past there have been many motor speed control circuits which attempt to regulate the speed of the motor, these circuits, by and large, are heavily dependent on the supply voltage for the regulation. Further, because of the criticality of the speed control in the above applications, it has been found that these circuits must be temperature compensated in order to maintain a constant motor speed. Additional temperature compensation circuits as well as additional power supply regulating circuits not only involve increased drain on the battery, but also result in additional active components which raise the cost of the motor control circuit.
It is a feature of the subject circuit that temperature effects and voltage source variation effects are cancelled from the subject circuit. Further in the discriminator-comparator section of the circuit there is derived a difference current which is determined by the difference between the desired speed and the actual speed of the motor. This current is converted to a voltage and applied to the motor such that the motor speed is independent of temperature and power supply voltage variations. It is another feature of this circuit that this difference current is in fact very small.
A specialized output circuit is shown which not only effectively converts this very small error current signal into a useable voltage control signal, but also utilizes a common-emitter drive configuration. This common-emitter drive configuration is important because it does not lower the voltage delivered to the motor to any great extent. The importance of this latter function is as follows: Assume, for instance, that the motor is to be run from a 12-volt source and assume that the motor will not operate at all at a voltage below, for instance, 9.0 volts. Then assuming that the 12-volt source can at times dip to as low as 10.5 volts, it will be appreciated that any output circuit which drops this voltage by any more than 1.5 volts will not be acceptable. Because of the particular configuration of the subject output circuit, the supply voltage is dropped solely by the saturation voltage of the output transistor. The common-emitter drive is achieved by connecting the motor between the power supply and the collector of an output transistor which has an emitter coupled to ground. The voltage across the terminals of the motor is stabilized by a negative feedback circuit. This circuit consists of a feedback path developed between the collector of the output transistor and the input to the output circuit via a transistor. The primary purpose of this feedback path is to insure a low impedance node at the collector of the output transistor which in turn insures a voltage node drive for the DC motor. Additionally, this negative feedback circuit functions to limit or counteract the input current to the output circuit so as to stabilize the circuit. What has been accomplished by this particular output circuit is to convert a control current into a stabilized control voltage. Because the output circuit does not significantly absorb much of the supply voltage, the motor can be made to operate even when low voltage swings occur in the power supply.
The protection circuit is a current-limiting circuit which shunts drive current away from the aforementioned control transistor in response to any overheating condition. In addition the protection circuit limits the amount of current that can be drawn through the output circuit by also shunting current away from the drive to the output transistor. The protection circuit in general is powered by voltages which to a first approximation are independent of the supply voltage. These voltages are used as reference voltages such that the temperature and current protection is independent of supply voltage fluctuations. These power supply independent voltages are generated by a constant current from a constant current source through a zener diode to ground.
In the general operation of the circuit the speed of the motor is sensed by a tachometer circuit which generates a sinusoidal signal. This sinusoidal signal is fed, without clipping or pulse-shaping, to the discriminator-comparator circuit which generates a pulse of current each time the sinusoidal input signal goes negative. Over a period of time the number of such pulses generated by this portion of the circuit corresponds to the frequency of the tachometer circuit or the speed of the motor. These current pulses are subtracted from a reference current and the difference is integrated over time so as to develop a DC error current which is supplied to the base of an NPN transistor such that the current flowing in the collector of this transistor is an error current proportional to the difference in speed between the actual speed of the motor and that corresponding to the reference current.
The average current carried by the pulses generated during negative swings of the tachometer signal is the feedback current which represents the speed of the motor. Because the feedback current is subtracted from the reference current, the circuit is insensitive to variations of supply voltage or temperature as the reference and feedback currents are made to have a similar dependence on temperature and supply voltage. This reference current is preset by an external resistor. The value of this resistor as well as the value of an external capacitor determine the speed of the motor. This error current is delivered to the input of the output circuit. The servo system operates ideally such that when the current difference is zero the motor is running at the desired speed. In response to this error signal the output circuit drives the base of an output transistor connected between the motor and ground. The motor is connected between the V+ power supply and the collector of this output transistor. When the error signal increases the output transistor increases in conductivity. The voltage at the collector of the output transistor is thus lowered with respect to V+. This increases the voltage across the motor to speed the motor up. The collector of the output transistor is made a low impedance node by the use of negative feedback circuitry which couples the collector of the output transistor to the input of the output circuit. This negative feedback circuitry also stabilizes the output circuit while at the same time providing a voltage, rather than a current drive to the motor. It is well known that electric motors are more accurately controlled by voltage driving systems as opposed to current driving systems.